Baltar cannot be a Cylon, it just doesnt make sense and is not possible. If he was a Cylon then why was Six needed? if he was a Cylon then why didnt Six know? He is not a Cylon - it makes absolutely no sense. Baltar is a human, his character was written that way and they had much fun playing with him as a human - it wouldnt make sense nor be effective to make him a Cylon - if they do then thats just poor writing for shock value.
I always thought Ellen was a Cylon, mostly because of that episode when they had someone (was it zarek?) captive and then inexplicably he was let out. It hinted that Ellen would let him out, but it was also what the Cylons wanted.
Also Adama (not sure which) is maybe a Cylon. Remember when Leobin told Kara that Adama was a Cylon, and the Laura threw him out the airlock. So far Leobin hasnt lied about anything. (also remember the grey haired Commander Adama in Baltars dreams that drowned the Cylon child)
Also, Kara cannot be a Cylon, otherwise how did she have a child? It doesnt make sense. End of story.
Someone in this thread was talking about their hopes for a Cylon that wanted the humans to survive, well there already is one - Six. The Six in Baltar's head always wanted humans to survive, her plan was never to kill them off. They need to serve God's purpose.
On a side note i really want the Six in Baltar's head to come back, she was commanding, authorative, seductive, and strong - the other Six is compassionate and seemingly weak. I want the strong one back.
Laura could be a Cylon, that'd make for an interesting twist, but im not sure they'd be able to pull that off well.